A beam angle optimization technique for proton pencil beam scanning treatment planning of lower pelvis targets

Abstract

The optimal proton pencil beam scanning (PBS) beam angle was calculated for a population of patients with lower pelvis targets. 10 patients were planned with a single PBS beam in the left lateral, right lateral, and posterior directions. A beam direction was considered to be optimal if it satisfied two metrics: shortest path length and least Hounsfield Unit (HU) variation. To determine these metrics, a ray-trace approach was adopted where the length of a ray represented the path length and the variation of HUs across the ray represented the HU variation. A Kolmogorov–Smirnov test determined the normalcy of the path length and HU variation at the 95% confidence level. Results showed that both the path length and HU variation were normally distributed. The path length was shortest for the posterior beam, and the left lateral beam had the least HU variation. Combining both optimization metrics, the posterior beam was more optimal since i) the path length was much shorter than the lateral beam’s path length, and ii) the HU variation along the posterior beam’s path length was only slightly less homogeneous than the HU variation along the lateral beam’s path length, but statistically similar. This study found that a posterior beam is optimal for lower pelvis targets when a single PBS beam is used.